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Module 2 - Planetary Formation and Differentiation

A Moment Frozen in Time

On May 19th, 2005, NASA's Mars Exploration Rover Spirit captured this stunning view as the Sun sank below the rim of Gusev crater on Mars. This Panoramic Camera (Pancam) mosaic was taken around 6:07 in the evening of the rover's 489th martian day, or sol. Spirit was commanded to stay awake briefly after sending that sol's data to the Mars Odyssey orbiter just before sunset. This small panorama of the western sky was obtained using Pancam's 750-nanometer, 530-nanometer and 430-nanometer color filters. This filter combination allows false color images to be generated that are similar to what a human would see, but with the colors slightly exaggerated. In this image, the bluish glow in the sky above the Sun would be visible to us if we were there, but an artifact of the Pancam's infrared imaging capabilities is that with this filter combination the redness of the sky farther from the sunset is exaggerated compared to the daytime colors of the martian sky. Because Mars is farther from the Sun than the Earth is, the Sun appears only about two-thirds the size that it appears in a sunset seen from the Earth. The terrain in the foreground is the rock outcrop "Jibsheet", a feature that Spirit has been investigating for several weeks (rover tracks are dimly visible leading up to "Jibsheet"). The floor of Gusev crater is visible in the distance, and the Sun is setting behind the wall of Gusev some 80 km (50 miles) in the distance. This mosaic is yet another example from MER of a beautiful, sublime martian scene that also captures some important scientific information. Specifically, sunset and twilight images are occasionally acquired by the science team to determine how high into the atmosphere the martian dust extends, and to look for dust or ice clouds. Other images have shown that the twilight glow remains visible, but increasingly fainter, for up to two hours before sunrise or after sunset. The long martian twilight (compared to Earth's) is caused by sunlight scattered around to the night side of the planet by abundant high altitude dust. Similar long twilights or extra-colorful sunrises and sunsets sometimes occur on Earth when tiny dust grains that are erupted from powerful volcanoes scatter light high in the atmosphere.

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Atmosphere Atmosphere refers to a layer of gasses surrounding a planet or other body that is held in place by the gravity of that body. More massive planets have stronger gravitational attraction and thus can hold onto thicker, more gas-rich atmospheres. Conversely, smaller planets like Mars, or even moons like Saturn's Titan have thinner atmospheres.

Planetary atmospheres initially form by outgassing of the planet’s interior. Rocks and molten rocks (magmas) have gasses (like carbon dioxide or water) dissolved within them. Release of the gasses may be gradual or catastrophic, such as during a volcanic eruption or impact event.
Earth
The make up of the Earth’s atmosphere is a byproduct of biological activity. Photosynthetic organisms have harvested light and carbon dioxide and created an oxygen-rich atmosphere. Earth’s atmosphere consists mainly of nitrogen (78.08%), oxygen (20.95%), argon (0.93%), and carbon dioxide (0.039%). There is also variable amounts of water vapor (~1% at sea level and ~0.4% averaged over the whole surface). In addition to providing us with breathable air, the atmosphere protects life by absorbing ultraviolet solar radiation and reducing temperature extremes from day and night. Earth also has an active hydrosphere, which refers to the combined mass of water found on, under, and over the surface of a planet in the form of oceans, ice caps, and groundwater.

Venus and Mars
For comparison, atmospheres of Venus and Mars are mainly composed of carbon dioxide and small quantities of nitrogen, argon, oxygen, and sulfur dioxide.

Rovers snapping sunset images from the surface of Mars see blue sunsets, which are a product of the scattering of light by dust in the Mars atmosphere and its dry, carbon dioxide-rich composition

The atmosphere of Venus is notably thick with an atmospheric pressure at the surface equivalent to pressures of nearly 1 kilometer under the Earth’s oceans. The thick, carbon dioxide-rich atmosphere of Venus is credited with its runaway greenhouse effect and creating surface temperatures of at least 462˚C.
Mercury
The thickness of a body’s atmosphere is also influenced by temperature. Tiny Mercury has the thinnest atmosphere of the eight planets circling the Sun. Mercury is constantly bombarded by solar weather. Much of the hydrogen and helium in Mercury’s thin atmosphere originated from the Sun. In addition, Mercury’s negligible atmosphere contains oxygen and the volatile metals sodium and potassium.